1. Field of the Invention
This invention relates to heat transfer control apparatus and, more particularly, pertains to a variable heat transfer device, or thermal switch, capable of varying its thermal resistance to the transfer of heat from a warmer body to a colder body in response to changes in the temperature of one or the other of said bodies.
2. Description of the Prior Art
As is well known, there are three basic ways of transferring heat from a heat source to a heat sink, viz, convection, conduction or radiation. In convection, a fluid, either a gas or a liquid, picks up heat from a source and transfers the heat through movement of the fluid and contact with a heat sink. In radiation, energy is propagated through space or material in the form of electromagnetic waves or corpuscular emissions. Conduction occurs when one solid body is in contact with another or where an intermediate body of a gas, a liquid or another solid material is interposed between the two bodies so as to act as a heat conducting medium.
For certain applications, such as, for example, in space vehicles or in a vacuum generally, convection is not a readily available heat transfer mechanism. The most common method by which heat is dissipated from space vehicles is by radiation from the outer surface thereof. Often, however, there are components and systems within such vehicles which are required to be maintained at substantially constant temperatures despite the operation of factors which tend to cause temperature variations. Dissipation of heat by radiation alone usually will not satisfy this requirement for local temperature stability. In these cases, heat is required to be transferred to the heat sink by conduction in a manner which produces the local temperature stability.
A thermal switch is a device which provides a variable thermal resistance, or conductance, and is generally temperature actuated. The thermal actuation devices most commonly found are, for example, bimetallic devices, expanding fluids, and materials which expand when undergoing a phase change. The latter include, for example, certain waxes such as those which are commonly used in automobile thermostats. Many thermal switches use metal conductors as the actual heat transfer medium although some switches use heat pipes for thermal conduction.
A heat pipe comprises a casing of heat conducting material and an interior sealed chamber containing a heat transfer fluid, such as water, which exists in both its liquid and vapor phase in the chamber. The fluid undergoes a closed thermodynamic cycle involving vaporization or boiling of the liquid in the heat input or evaporator section of the heat pipe and flow of the resulting vapor to the heat output or condenser section of the pipe. The fluid condenses to the liquid phase in the condenser section whereupon it is returned to the evaporator section by capillary flow in a wick saturated with the liquid. The cycle is then repeated.
A thermal switch using a bimetallic device is disclosed in Riordan, U.S. Pat. No. 3,225,820 issued Dec. 28, 1965. A thermal connection between a component package generating heat and a heat sink is provided by and through a bimetallic element. The bimetallic element responds to changes in the temperature of the heat sink to vary the effective contact area between the bimetallic element and the heat sink. As is well known, thermal conductance and contact area are related to each other directly.
Another example of a thermal switch using a bimetallic device is disclosed in Kelly, U.S. Pat. No. 3,302,703 issued Feb. 7, 1967. A gap between a heat source and a heat sink is enclosed in a sealed housing substantially filled with a low-thermal-conductivity gas such as carbon dioxide. The sealed housing includes a reservoir in the form of a bellows which holds a supply of a liquid thermal conductor such as mercury. Pressure on the bellows causes the mercury to be forced into the gap between the heat source and heat sink while the gas is displaced into a second reservoir. A bimetallic element responsive to temperature changes in either the heat source or the heat sink produces changes in the pressure on the bellows, thereby varying the thermal conductivity across the gap.
In Townsend, U.S. Pat. No. 3,390,717 issued July 2, 1968, there is a discussion of the mechanism of heat transfer from one solid metal plate to another in a vacuum. The thermal resistance is inversely proportional to the area of contact between the two plates. The rate of heat transfer from one plate to the other is varied by changing the pressure tending to force the plates together. This phenomenon occurs because even highly polished plates will have valleys and ridges on their juxtaposed surfaces. These ridges and valleys cause spaces and voids between the plates where they are not in actual contact. This effect is discussed further in Sheppard et al, U.S. Pat. No. 3,721,102 issued Mar. 20, 1973. It is stated therein that the thermal conductivity of a physical contact between two surfaces in a hard vacuum is proportional to the true surface area of the contact and, therefore, is inversely related to the yield strength of the contacting materials and directly related to the normal force between the contacting surfaces. An approximate mathematical relationship between pressure, yield strength, and thermal conductance derived empirically is given in Aron and Colombo, ASME Paper No. 63-WA-196, "Controlling Factors of Thermal Conductance Across Bolted Joints in a Vacuum Environment", Winter Annual Meeting of The American Society of Mechanical Engineers, Philadelphia, Pa., Nov. 17-22, 1963. The thermal conductance at a bolted joint is given by a constant exponential of the ratio of pressure to yield strength of the particular metal at a heat conducting interface.
In Myers, U.S. Pat. No. 3,463,224 issued Aug. 26, 1969, there is disclosed a thermal switch comprising a bellows filled with a heat expandable fluid. The bellows is in continual thermal contact with a heat sink. A temperature rise of the heat sink causes the expandable fluid to extend the bellows, whereupon thermal circuits from the source to the sink are closed through metal conductors.
A thermal switch similar to the one disclosed in Myers, supra, but employing a heat pipe for thermal conduction is disclosed in Cline, U.S. Pat. No. 3,399,717 issued Sept. 3, 1968. The switch comprises a pair of sealed bellows disposed in concentric relationship. The inner bellows is filled with a heat expandable fluid while the expandable chamber space between the inner and outer bellows contains a two phase fluid and a capillary element or wick to form the heat pipe. A plate at one end of the bellows assembly is in contact with a heat source. A raised temperature at the heat source causes expansion of the heat expandable fluid which, in turn, expands the bellows bringing a plate at the opposite end thereof into contact with a heat sink.
As a general proposition, it may be said that the prior art devices discussed hereinabove have relatively low gain. That is because bimetallic elements and bellows, used separately or in combination, require relatively large temperature changes to produce appreciable displacements or forces for actuation.
3. Prior Art Statement
The most pertinent prior art discovered by applicant relative to this invention is listed herewith:
(1) Reinke, "Variable Heat Conductor", U.S. Pat. No. 3,478,819 issued Nov. 18, 1969;
(2) Sheppard et al, "Method and Apparatus for Cooling a Load", U.S. Pat. No. 3,721,101 issued Mar. 20, 1973.
The patent to Reinke discloses a thermal switch having a cylindrical chamber formed in a heat source. A first portion of this chamber contains a thermally responsive element which may be a material which changes state and expands at a selected temperature. Expansion of the thermally responsive element forces a heat conductor piston to extend from the cylindrical chamber in the heat source into a cup-shaped chamber in a heat sink. So extended, the piston is in thermal contact with both the heat source and the heat sink. The thermal resistance between the piston and the heat source is substantially constant. However, the area at the contact made between the piston and the walls of the cup-shaped chamber in the heat sink is proportional to the extension or depth of penetration of the piston into the cup-shaped chamber. There is thereby produced a variable thermal resistance between the heat source and heat sink.
The patent to Sheppard et al discloses a controllable heat flow path between a first heat station and a second heat station. A flexible copper strip is in thermal contact with and affixed to the first heat station at one end thereof. At the other end thereof, the copper strip is disposed to be engaged and disengaged with the second heat station by means of a rod which extends externally from the primary housing of the device. The pressure at the thermal contact between the copper strip and the second heat station is controllable by applying a variable force to the externally disposed end of the rod.